Scintillation detectors are used in the oil industry for deep well logging. Typically, the detector is lowered into a bore hole in the earth for analysis of surrounding rock strata. The detectors are relatively small in diameter so that they may be accommodated in special logging equipment lowered into the bore hole. A common practice is to measure while drilling (MWD). For MWD applications the detector must be capable of withstanding high temperatures and also must have high shock resistance.
For many years there have been used well logging detectors which comprise relatively small diameter metal tubes or casings in which sodium iodide crystals (or other scintillation crystals of choice) are enclosed or encapsulated. The casing is closed at one end by a plug or cap and at the other end by an optical window which permits radiation induced scintillation light to pass out of the detector for measurement by a light sensing device such as a photomultiplier tube. Usually a hermetic seal is provided to prevent the crystal and other internal parts of the assembly from being damaged by exposure to the detector's environment. For example, sodium iodide crystals are highly hygroscopic and easily damaged if exposed to moisture.
The crystal in these prior art detectors has a diameter less than the internal diameter of the tube to provide an annular space which is filled with a highly reflective powder such as aluminum oxide powder. The reflective powder prevents the crystal from moving radially within the casing and further functions to reflect scintillation light back into the crystal for ultimate passage out of the crystal through the optical window. Usually an optical coupling is provided between the end of the crystal and the window and the crystal is biased by a spring or other resilient means towards the optical coupling at the window.
In U.S. Pat. No. 4,158,773 there is disclosed a scintillation detector wherein a silicone rubber sleeve is interposed between the crystal and the tubular casing for purposes of shock absorption. The sleeve has a multiplicity of closely arranged tapered protrusions radiating toward and engaging the crystal to cushion shocks. The radially inwardly directed protrusions define therebetween spaces which are filled with aluminum oxide powder. The silicone rubber composition of the sleeve may include aluminum oxide which enables the rubber sleeve to function as a light reflector. In an alternative embodiment the reflective powder is omitted and the cylindrical surface of the crystal is highly polished and surrounded by a thin cover layer or sheet of white paper or plastic, prefereably white Teflon (polytetrafluoroethylene). In turn, the thin cover layer is surrounded by the shock absorbing sleeve.
In U.S. Pat. NO. 4,383,175 there is disclosed another scintillation detector. In this detector a lubricating layer is interposed between the crystal and a surrounding layer of light-reflecting packing such as powdered aluminum oxide. The preferred lubricating layer is said to be a skived sheet of Teflon (polytetrafluoroethylene). This design is said to provide for reduction of friction between the crystal and the packed powder during temperature induced differential expansion of the crystal and casing.